Current Public Land Mobile Networks (PLMN) typically comprise a radio access network and a core network. The radio access network is responsible for providing wireless access to network users, whilst the core network is responsible, inter alia, for subscriber service access control and subscriber roaming. In the case of 2G or GSM network, a typical PLMN comprises circuit switched (CS) core network for handling voice calls and a packet switched (PS) core network for handling data. A 3G or UMTS network similarly comprises PS and CS core networks, whilst a 4G or LTE network comprises only an Enhanced Packet Core (EPC) core network. In addition, service networks may co-exist with the PS and CS core networks. One such service network is the IP Multimedia Subsystem (IMS) network. The IMS network may also be considered as a core network. In practical implementations, subscribers may roam between different radio access networks, and services may be switched between core networks, e.g. a voice call may be switched from a 3G radio access network and CS core network to a 4G network, with the same IMS core network remaining in control of the session.
PLMN operators are always keen to reduce network signalling load for a given subscriber volume, to increase network resilience to failure, and to reduce the complexity and number of specialist network nodes. With this in mind, the 3GPP organisation has defined a so-called Data Layered Architecture (DLA) that splits the traditional node and network architecture introduced for GSM, UMTS and EPC in two halves; an application front end (Application-FE) and a Data back end (Data-BE). This is illustrated in FIG. 1. The Data-BE provides a highly available and geo-redundant solution to the Application-FEs that simplifies the design of applications. DLA also simplifies the routing in the core networks since any node can, if it is built as a stateless FE, serve any subscriber. The principles and advantages of DLA can be summarised as follows:                Simplified routing to Application FEs in Core Networks                    Any Application FE can serve any subscriber                        Data & Repository Consolidation                    Several applications use the same backend (repository consolidation)            Data de-duplication by allowing several applications to access the same data object (data consolidation)                        Scalable and Highly Available system                    Backend provides HA and Geo Redundancy                        Simple scaling of Application Front Ends        Simplified Application Design                    Provisioning into back end database            Dynamic allocation of subscribers to Application FEs            Allows for stateless Application FE design            Data Availability provided by the Backend                        
Consider the Home Location Register (HLR) that is implemented in 2G and 3G core networks and that acts as a central database for subscriber information. The HLR stores details of subscription information generated issued by the network operator. Typically, a network will comprise a number of HLRs, each of which is assigned to a group of network subscribers. An HLR provides subscriber information to other core network nodes, such as the 2G MSC. In the traditional architecture, core network applications, such as the MSC, must identify and route signalling to the correct HLR (i.e. the HLR currently serving a given subscriber). According to the DLA architecture, an HLR-FE can serve any subscriber (i.e. all network subscribers) and network routing becomes very simple: the advanced Data-BE performs routing for the applications. Other advantages of DLA include Data and Repository Consolidation allowing databases and data-object sharing, and simplified provisioning (and removal) of subscribers within the system.
While the Data-BE is designed to provide a highly available, resilient and redundant service to the Application-FEs, extreme failure conditions may occur. If the Data-BE becomes unavailable, one problem that will occur in a mobile network is that the Core Network nodes (e.g. MSC/VLR and SGSN) will run out of authentication triplets (2G) and authentication vectors (3G). These are needed for the authentication procedure between the mobile terminal or station (MS) and the core network for mobility management signalling such as IMSI Attach and Location Update. They are also required for procedures involving users such as originating call attempts and SMS sending.
The consumption rate of authentication vectors and triplets is governed by parameter settings instructing a core network node how often authentication shall be performed (e.g. for every 5th procedure). When they have been consumed, core network nodes will start requesting new authentication data. If the Data-BE is unavailable, the rejection of authentication data will trigger re-transmission of requests that will flood the core network and the Application-FEs. Changing the parameter settings that govern how often authentication shall be performed at a given node is a manual configuration procedure and will only temporarily solve (i.e. delay) the problem. Turning off authentication in the event of a Data-BE failure will of course severely impact the network security.